Since a liquid crystal display (LCD) device has characteristics of light weight, thinness and low power consumption, the LCD device has been widely used as a substitute for a display device of cathode-ray tube type. The LCD device includes first and second substrates that face each other. A liquid crystal layer is interposed between the first and second substrates. The LCD device uses optical anisotropy and polarization properties of liquid crystal molecules to display images. The LCD device includes a switching element, a pixel electrode, a common electrode, a color filter and so on. Particularly, the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images.
FIG. 1 is an exploded perspective view of a conventional liquid crystal panel. As shown in FIG. 1, the liquid crystal panel includes an array substrate 10, a color filter substrate 20, and a liquid crystal layer 30. The array substrate 10 and the color filter substrate 20 face each other. The liquid crystal layer 30 is interposed therebetween.
The array substrate 10 includes a first substrate 12, a gate line 14, a data line 16, a thin film transistor (TFT) “T”, and a pixel electrode 18. The gate and data lines 14 and 16 are formed on the first substrate 12 and cross each other to define a pixel region “P”. The TFT “T” is formed at a crossing portion of the gate and data lines 14 and 16. The pixel electrode 18 is formed in the pixel region “P” and connected to the TFT “T”.
The color filter substrate 20 includes a second substrate 22, a black matrix 25, a color filter layer 26, and a common electrode 28. The black matrix 25 is formed on the second substrate 22 and has a lattice shape. The black matrix 25 corresponds to a non-display region of the first substrate 12. The non-display region of the first substrate 12 includes the gate and data lines 14 and 16 and the TFT “T”. The color filter layer 26 corresponds to the pixel region “P” and has one of red, green, and blue colors “R”, “G”, and “B”. Namely, the color filter layer 26 includes red, green and blue color filter patterns 26a, 26b and 26c. The common electrode 28 is formed on the black matrix 25 and the color filter layer 28. The common electrode 28 generates an electric field with the pixel electrode 18 such that the liquid crystal layer 30 is driven by the electric field.
Though not shown, a seal pattern is formed along edges of the first and second substrates 12 and 22. The seal pattern prevents the liquid crystal layer 30 from overflowing. In addition, first and second alignment layers may be formed between the first substrate 12 and the liquid crystal layer 30 and between the second substrate 22 and the liquid crystal layer 30. A polarization plate may be formed on an outer surface of one of the first and second substrates 12 and 22. A backlight assembly is formed on a rear side of the first substrate 12 to apply light into the liquid crystal panel.
Generally, a glass plate is used for the first and second substrates 12 and 22. However, recently, a flexible plate, such as a plastic plate, is used for the first and second substrates 12 and 22 because the flexible plate is light and flexible. Unfortunately, since a process of fabricating an array substrate is performed under a temperature higher than about 200° C., it is very difficult for the flexible plate to be a substitute for the glass plate. So, the array substrate is made of the glass substrate, and the color filter substrate is made of the flexible substrate.
When processes of forming a metal layer, a gate insulating layer, a passivation layer are performed under a temperature lower than 200° C., a property of the TFT does not deteriorate. However, when a semiconductor layer is made of amorphous silicon under such a lower temperature, a property of the TFT does deteriorate. To resolve these problems, a method of fabricating the array substrate under a temperature lower than about 200° C. by forming the TFT using an organic semiconductor material is suggested.
FIG. 2 shows a process of fabricating a semiconductor layer using an organic semiconductor material and a shadow mask by evaporating. First, a gate electrode 52 and a gate line (not shown) are formed on a substrate 50 by depositing and patterning a metal layer (not shown). A gate insulating layer 53 is formed on the gate electrode 52 and the gate line (not shown) by depositing an organic insulating material. Next, a semiconductor layer 54 corresponding to the gate electrode 52 is formed on the gate insulating layer 53 by depositing and evaporating an organic semiconductor layer (not shown) using a shadow mask 56. In this case, there are limitations regarding a distance w1 between shadow mask patterns and a width w2 of the shadow mask patterns. For example, the shadow mask patterns should have the width w2 greater than 50 micrometers.
When the substrate is made of the glass plate, the semiconductor layer is formed by depositing and patterning silane (SiH4) using a photoresist layer and a patterning mask. Silane is deposited by a method of chemical vapor deposition (CVD). However, it is difficult for the organic semiconductor material to be deposited and patterned by the above mentioned process. Since the organic semiconductor material is a powder type, it is difficult to deposit the organic semiconductor material by the CVD method. Moreover, when the organic semiconductor material contacts the photoresist layer including moisture and an acidic or basic liquid used to develop the photoresist layer, a property of the semiconductor layer deteriorates. Accordingly, the semiconductor layer of the organic semiconductor material is formed by evaporating using the shadow mask, instead of pattering using a patterning mask. However, as mentioned above, since there are some limitations regarding the shadow mask, it is difficult to produce the semiconductor layer used for the display device having a precision structure and a high resolution.
The organic semiconductor material is divided into a high molecular weight organic semiconductor material and a low molecular weight organic semiconductor material. Since the low molecular weight organic semiconductor material has better properties, it is used for the semiconductor layer as a substitute of the amorphous silicon. However, since the low molecular weight organic semiconductor material is only slightly soluble in an organic solvent and alcohol, it is difficult to convert the low molecular weight organic semiconductor material into a liquid phase.
To resolve these problems, the TFT of a bottom gate type is explained referring to FIG. 3. FIG. 3 is a cross-sectional view of an array substrate for an LCD device having an organic TFT of a bottom gate type and a bottom contact type.
First, the gate electrode 62 is formed on the substrate 60 by depositing and pattering a metallic material. And the gate insulating layer 63 is formed on the gate electrode 62. The source and drain electrodes 64 and 66 separating from each other are formed on the gate insulating layer 62. The source and drain electrodes 64 and 66 correspond to both ends of the gate electrode 62, respectively. Then, the low molecular weight organic semiconductor material layer (not shown) is evaporated to be deposited on the source and drain electrodes 64 and 66. The organic semiconductor layer 68 is formed on the source and drain electrode by patterning the low molecular weight organic semiconductor material layer (not shown). Since the low molecular weight organic semiconductor material layer (not shown) is formed on the uppermost layer of the TFT Tr, it is not exposed by the organic solvent and alcohol. Thus the TFT has a bottom gate type and a bottom contact type.
However, in the case of the bottom gate type and the bottom contact type, there is a high contact resistance. Accordingly, the TFT Tr has deteriorated properties.
Conversely, in the case of the bottom gate type and a top contact type, the TFT has preferable properties. However, since the low molecular weight organic semiconductor material is exposed by the organic solvent included in an etchant, it does not have suitable properties for the semiconductor layer.
As shown in FIG. 4, a method of fabricating the TFT of the bottom gate type and a top contact type is suggested to resolve the above-mentioned problem. As shown in FIG. 4, the gate electrode 72 and the gate insulating layer 73 are formed on the substrate 70. Then, the organic semiconductor layer 78, the source electrode 74 and the drain electrode 76 are formed on the gate insulating layer by using a shadow mask 80. However, a distance “d” between the source and drain electrodes 74 and 76, that is, a distance of a channel, is as large as several micrometers. Accordingly, a size of TFT Tr increases, and an aperture ratio and resolution of the LCD device are deteriorated.